1. Field of the Invention
The present invention relates to gas turbine engines and, in particular, to a gas turbine engine having a rotor casing with a contoured step entry to a turbine wheel.
2. Prior Art
The efficiency of a turbine section in a gas turbine engine is generally determined by how effectively the turbine can convert the kinetic energy from the hot gases exiting the combustors into shaft horsepower. In the past, maximum turbine efficiency required minimum clearance between the rotating blade tips and the rotor casing surrounding the turbine blades and vanes. If, however, the clearance between the blade tips and the casing was too tight, there was a potential of interference between the two, whereas, if the clearance was too wide, a loss of efficiency resulted by flow of the gases between the blade tips and the casing rather than impacting upon the turbine blades.
In one type of procedure used in the prior art, the casing was provided with a trench into which the tips of the blades would extend. In yet another method, as disclosed by U.S. Pat. No. 1,554,052 by Weidehoff, covers were used between vanes and blades to prevent fluid from bypassing the blades. In yet another method, as disclosed by U.S. Pat. No. 4,311,431 by Barbeau, labyrinth seals are positioned between static shrouds and rotating shrouds on the blades to reduce the leakage of hot gases through the shroud clearance space. Also disclosed by Barbeau is the use of compressed air as a thermal energy loss barrier.
A problem arises in presently available gas turbine engines in that energy is lost in the turbine section of the engines because of flow bypass through the area between the blade tips and turbine case.
A further problem arises in presently available gas turbine engines in that performance efficiency is too sensitive to tip clearance with the rotor casing.
A further problem arises in presently available gas turbine engines in that overall size reduction of gas turbine engines have increased the proportion of turbine efficiency loss due to tip clearance.